Reinvestigation of the LuFeO3(ZnO)m Homologous Series: Insights from Charge Distribution Analysis on the Effect of the Coordination Polyhedra Shape on the Cation Distribution

Abstract

A previous report on the homologous series LuFeO3(ZnO)m (m=1, 4, 5, 6) is reanalyzed in terms of the charge distribution (CD) approach, an alternative to the bond valence method, which exploits the geometry of the coordination polyhedra. It is shown that in the m=1 member the octahedral site is not well balanced; besides, members m=4, 5, 6 are well balanced only assuming that Fe3+ avoids the cation site next to Lu. The main reason for these effects is likely in the shape of the trigonal bipyramids hosting Fe and Zn. The polyhedron next to the Lu site is highly distorted, with the three basal M–O distances midway between those of the two apical ones. This geometry, approaching the tetrahedral shape, is more suitable for hosting Zn than Fe; besides, the short apical M–O distance results in enhancement of the intercationic (Lu–M1) repulsion. For members m>1 such repulsion is likely avoided by distributing the trivalent cation into the m-1 trigonal bipyramids not directly bound to the octahedron. The m=1 member contains only one trigonal bipyramid and cannot thus adopt the same scheme of cation distribution. As a result, Lu is probably shifted outside the center of symmetry. A new refinement of the m=1 member, based on the original intensities, shows that Lu is displaced by ±0.14 Å from the center of symmetry. With respect to the refinement in the central atom model, a lower R factor, a lower Fourier difference and thermal parameters have been obtained. The results of the present research suggest the Lu–Fe3+ repulsion, related to the asymmetry of the trigonal bypiramidal coordination of Zn/Fe, as a possible cause of the deviation from the average structure.